The present invention relates to an improvement in gallium phosphide (GaP) single crystals manufactured by the well-known or conventional liquid encapsulation Czochralski pulling method "See `Pulling of Gallium Phosphide crystals in liquid encapsulation` by S. J. Bass and P. E. Oliver in Journal of Crystal Growth 3-4 (1968) 286-290", (hereinafter to be referred to as the LEC method) with novel improvements. More particularly, it relates to GaP single crystals with low defect density doped with a strongly reducing impurity such as silicon and a method of manufacturing such crystals.
In the manufacture of GaP single crystals, employment of the LEC method has made it possible to obtain large sized crystals. However, the environment for growth of a GaP crystal is such that the temperature of the melt is required to be high, such as approximately 1500.degree. C., and in addition, the crystal is pulled up in boron oxide (B.sub.2 O.sub.3) encapsulant having a temperature gradient of 200.degree. C..about.500.degree. C. in a high pressure gas of about 50 kg/cm.sup.2. Consequently, the crystal is subjected to heavy thermal stress, so that plastic deformation of the crystal takes place and a great number of dislocations are introduced or multiplied. If the (111)B surface of a GaP crystal obtained by the LEC method in this manner is examined with RC etchant (see "Effect of dislocation on green electroluminescense efficiency in GaP grown by liquid phase epitaxy" by W. A. Brantley and others in Journal of Applied Physics, Vol. 46, No. 6, June 1975, p. 2629), the dislocation etch pit density (hereinafter to be referred to briefly as D-EPD) is usually found to be 1.about.10.times.10.sup.5 cm.sup.-2. Beside these dislocation etch pits, a large number of the so-called saucer pits, which are small pits shaped like shallow saucers, are also observed, and sometimes their density reaches the order of 10.sup.7 cm.sup.-2. Regarding this saucer pit (hereinafter to be referred to as S pit), a detailed report is made by T. Iizuka in "Etching Studies of Impurity Precipitates in Pulled GaP Crystals" (J. Electrochem. Soc.: SOLID STATE SCIENCE July 1971 Vol. 118, p. 1190). It is stated therein that S pits are probably due to precipitates related to the impurity used for doping or, in the case of undoped crystals, precipitates related to the residual impurities such as boron, silicon, carbon or oxygen.
Generally, there are many defects existing in LEC GaP as mentioned above. On the other hand, however, GaP single crystals having few defects are demanded for their application product, light emitting diodes (LED), to improve their light emitting efficiency. For example, it is stated in the "Effect of dislocations on green electroluminescence efficiency in GaP grown by liquid phase epitaxy" (Journal of Applied Physics, Vol. 46, No. 6, June 1975, p. 2629) by W. A. Brantly et al, that the electroluminescent efficiency of green LED is dependent on the dislocation density of the epitaxial layer and that, when it is not less than 10.sup.5 cm.sup.-2, the electroluminescent efficiency decreases greatly as the dislocation density increases. Also, since the D-EPD of the epitaxial layer about corresponds to the D-EPD of the substrate where the D-EPD of the substrate is in the order of 10.sup.5 cm.sup.-2, it can be seen that it is ultimately necessary to lower the defect density of the substrate and lower the D-EPD of the epitaxial layer. It is desirable that the D-EPD of epitaxial layer for green LED not be in excess of 1.times.10.sup.5 cm.sup.-2, and if possible, not in excess of 5.times.10.sup.4 cm.sup.-2.
One method employed for making LEC GaP single crystals which have few defects is a method wherein they are pulled from non-stoichiometric melts. For example, this is reported in "Defect Studies of GaP Crystals Pulled from Nonstoichiometric Melts: Dislocation and `Saucer` Etch Pits" (J. Appl. Phys. Vol. 43, No. 7, July 1972, p. 3141) by G. A. Roxgonyl et al, and crystals with D-EPD in the order of 10.sup.2 cm.sup.-2 and without S pits were obtained. However, because the density of gallium inclusion increases, the pulling speed is extremely slow and high yield of single crystals cannot be expected, and thus, the method in which pulling is done from non-stoichiometric melts is not suitable for industrial purposes as compared with the method in which pulling is done from nearly stoichiometric melts.
On the other hand, crystals with D-EPD in the order of 10.sup.4 cm.sup.-2 may be found among conventional sulphurdoped LEC GaP crystals. In many cases, however, the EPD of epitaxial growth layers on these substrates becomes in the order of 10.sup.5 cm.sup.-2 and this does not help toward attaining the object of decreasing EPD of epitaxial growth layer. This point was taken up, for example, in "Dislocation in the liquid phase epitaxial growth layer and LEC substrate of GaP", (Book of Lecture Papers 2, 24th Associated and Combined Lecture Meeting of the Japanese Society of Applied Physics, 1977, No. 28p - Q-4, p. 433), wherein small conical pits which are neither the so-called dislocation pits nor what are called S pits are observed in the substrate, and if their density is added to the ordinary D-EPD, it almost coincides with the D-EPD of the epitaxial layer. This means that a substrate of which the sum of D-EPD and small conical etch pit density is in the order of 10.sup.4 cm.sup.-2 is required to obtain an epitaxial layer of a low D-EPD in the order of 10.sup.4 cm.sup.-2.